But another of Trump's rivals, Ohio Governor John Kasich, attacked his plan and that of Carson as "fantasy tax schemes."

The Tax Foundation estimated that Trump's plan could cost trillions over the next decade.

Carson has proposed a 10% flat tax plan modeled on tithing.

Fighting talk: Trump challenges Kasich (Image: Getty)

"We're going to have a 10% tithe and that is how we are going to fund the government?" Kasich asked.

He added: "Folks, we've got to wake up. We cannot elect someone who doesn't know how to do the job."

That sparked Trump's ire, who accused Kasich of being a managing general partner of Lehman Brothers when it "went down the tubes."

He also said that Kasich's economic agenda in Ohio "got lucky with the fracking."

Attack: Kasich said Trump and Carson 'can't do the job' (Image: REUTERS)

Eleven Republican presidential candidates were on the stage in the main-event debate, which was focused on the economy.

The big difference between this debate and the last encounter, on September 16 is that Trump is no longer leading in polls in Iowa and, as of Tuesday, a nationwide poll conducted by CBS and The New York Times.

Leading: Carson is now number one in the polls (Image: REUTERS)

Carson, the retired neurosurgeon, has taken the lead, raising the prospect that he would be the focus of attacks by Trump and other opponents.

The debate also held big stakes for Jeb Bush, who has found himself languishing in the single digits after entering the race in June as the presumptive frontrunner, and Marco Rubio, who has seen a steady rise in the polls yet has recently faced criticism for missing votes in the Senate.

Slipping: Jeb Bush is languishing in the polls (Image: REUTERS)

That was evident in an exchange between Bush and Rubio. On Wednesday, the Sun Sentinel in Fort Lauderdale, Florida, called on Rubio to resign for missing so many Senate votes as he has waged his presidential campaign.

"It's evidence of bias in American media today," Rubio said, noting other contenders who missed votes yet did not earn the paper's wrath.

But Bush shot back that Rubio was endorsed by the paper. "This is a six-year term. You should be showing up for work."

Defence: Rubio was called on to resign his post (Image: Getty)

Rubio responded, "Someone has convinced you that attacking me will help you."

Perhaps standing the most to gain was CNBC, hoping for the record ratings that Fox News and CNN enjoyed in their coverage of the first and second debates, respectively.

The network has reportedly commanded a premium in ad rates, but it also was willing to alter its proposed debate format after Trump and Carson threatened to pull out if it ran longer than two hours.